Invitation to a Contemporary Physics (2004)

cosmic string String-like object with a high-energy density that could have been formed at the beginning of the universe. It can serve as a seed to accrete matter into galaxies.

cosmological constant A constant that can be incorporated into Einstein’s gravitational theory (general theory of relativity). It represents the energy density of a matterless universe. This number is observationally known to be either zero or very small, though present theories of elementary particle physics almost invariably predict a large value.

cosmological principle This principle states that the universe looks the same to every observer anywhere in the universe if local matter fluctuation is averaged out. It is the modern version of what Copernicus advocated some 500 years ago.

critical phenomena Singular thermodynamic behavior of a system close to a second-order phase transition. Example: Divergence of the magnetic susceptibility of a magnet at the critical point (Curie temperature).

degeneracy A state of matter characterized by extreme compactness or density in which the quantum e ects of indistinguishability (Bose or Fermi statistics) of identical particles become important. The degeneracy temperature is the temperature below which such e ects become observable. A system of identical particles is said to be degenerate when they occupy the lowest possible one-particle levels consistent with their respective statistics. For bosons, this means the same one-particle state, and for fermions, all oneparticle states up to a certain energy (the Fermi level), while all states with higher energies are empty. The pressure of degenerate matter is dominated by quantum e ects and is largely independent of temperature.

dielectric material A material in which an electric field can be sustained with a minimum of energy dissipation.

Doppler cooling Slowing down of an atom moving against a beam of (laser) light whose frequency is tuned slightly below that for resonant atomic absorption, i.e., red-detuned. Due to the Doppler-shift e ect, the red-detuned light photon is blue-shifted into atomic resonance and absorbed, giving in the process a momentum kick directed so as to retard the atom. The subsequent spontaneous re-emission is, however, undirected (isotropic). This results in a net retarding force slowing down the atom, and hence, the Doppler cooling.

Doppler shift The change in wavelength of waves as seen by an observer when the source and observer are in relative motion. When they are moving apart the wavelength increases and, conversely, when they approach the wavelength decreases. With sound, the Doppler e ect causes a shift in pitch, and with light, a shift in color of the source. Named for Christian Doppler.

electrochemical processes Processes in which an electric field is used to initiate a chemical reaction. For example, layers of nickel can be grown on a silver substrate by applying an electrode potential to the electrolyte Ni2+.

electron gas A collection of electrons which interact with each other weakly enough to be considered as practically free, subject only to the exclusion principle.

energy band A continuous range of energies in a solid in which there are allowed quantum states for electrons. Energy bands may be occupied or empty, and are separated from one another by gaps, which are energy regions where no states for electrons exist.

entropy A thermodynamic property which gives a measure of the degree of disorder of a macroscopic system.

event horizon The boundary of a black hole. Light or any signal emitted from inside cannot escape to the outside. Only light emitted outside can escape to infinity.

exclusion principle See Pauli’s exclusion principle.

fermion Particles having a half-integral intrinsic spin (measured in units of ). Quarks and leptons — all particles associated with matter and antimatter

— are fermions. Composite systems of odd numbers of fermions, such as baryons, 3He, 13C and so on, also behave as fermions. Fermions obey Fermi statistics, i.e., the Pauli exclusion principle, which forbids two identical fermions to occupy the same one-particle state in a given system.

Feynman diagram A pictorial representation of the quantum field theoretical predictions of particle and of many-body dynamics.

field A physical quantity that extends over space, to be contrasted with a particle, which is localized at a point in space. Examples are electric field, magnetic field and temperature distribution in a spatial region.

flavor The quality that distinguishes the di erent varieties of quarks — up, down, strange, charmed, top (or truth) and bottom (or beauty). This property is also used to distinguish the various types of leptons (electron, muon, tau).

fractal geometry Geometry of highly irregularly shaped objects having fractional dimensions (e.g., jagged coast-lines, sponges, foams). Studied first by Benoˆıt Mandelbrot.

fundamental interactions Forces acting between the basic constituents of matter, to be contrasted with e ective interactions, which are simplified forms of interactions between composite particles. The four known fundamental interactions are: the gravitational force, the electromagnetic force, the weak force and the strong force.

gamma rays A very energetic electromagnetic radiation, with wavelengths shorter than 10−11 m, or energies greater than 1 million electron-volts.

gas-phase condensation A precursor material is vaporized by some high-energy process (e.g., electric arc discharge, heating, laser ablation, irradiation by electrons) and the vapor is then condensed in a controlled inert-gas environment. Many metal, ceramic nanoparticles, fullerenes and tubulites are grown in this manner. In a related process, called physical vapor synthesis,

the precursor is heated by a plasma in an open atmosphere and the vapor is cooled by collisions with the atoms of a reactive gas.

gauge symmetry Invariance of the form of the dynamical equations with respect to certain (space-time dependent) changes of various fields.

gauge theories Theories of fundamental interactions based on gauge symmetry. In such a theory a spin-1 gauge field(s) is always present, whose longitudinal polarization is decoupled by the gauge symmetry. Particles having the same gauge quantum numbers always couple the same way to the gauge field(s).

general relativity Theory of gravitation developed by Albert Einstein. It gives the necessary equations to determine the geometry of spacetime in terms of a distribution of matter and energy.

globular cluster A gravitationally bound system of (typically 106) stars with a well-defined spheroidal or ellipsoidal shape. These stars are among the oldest known objects in our galaxy and are characterized by very low abundances of heavy elements.

gluon Massless boson of spin one and agent for the strong interaction of quarks. graviton Massless boson with a spin value of two, associated with the gravitational

force.

hadron Any strongly interacting particle, made up of quarks and antiquarks. Mesons and baryons are all hadrons.

Hawking’s radiation Radiation emitted by a black hole when a particle–anti- particle pair, produced near the event horizon, splits into two, the antiparticle falling through the event horizon and the particle escaping to infinity; discovered by Stephen Hawking.

Heisenberg’s uncertainty principle A general proposition in quantum mechanics that not all quantities in a quantum system may have simultaneously definite values. The uncertainty relations are mathematical formulas (due to Werner Heisenberg) that describe this irreducible level of uncertainty of certain pairs of dynamical variables when they are observed in identical situations.

Hertzsprung–Russell diagram The plot of stellar brightness versus surface temperature for stars (named in honor of Ejnar Hertzsprung and Henry Russell). Ordinary stars lie in a band called the main sequence; other types of stars are found in specific places on the diagram.

Higgs field A spin-zero field, some of whose components have non-zero values in the vacuum at a low enough temperature. This vacuum condensate is instrumental in giving non-zero masses to the gauge bosons (fields) and fermions.

Higgs boson Particle associated with a Higgs field which is not needed to provide the longitudinal polarization degree of freedom for a massive gauge boson; named after Peter Higgs.

holography Photography by reconstruction of light waves. The two parts of a laser beam, split into two by a half-silvered mirror, are directed at a photographic plate, one directly, the other after being scattered o the subject. The photographic plate records the interference pattern caused by the recombination of the two beams. When the hologram, as this photographic record is called, is illuminated by light from a laser identical in characteristics to the laser used for the recording, a three-dimensional image of the original object is reconstructed.

Hubble’s law The observation, first made by E. Hubble, that distant galaxies are receding from us with a velocity proportional to their distance.

inflationary universe A theory of the very early universe constructed to overcome certain di culties of the classical Big Bang theory. According to this theory, the universe underwent a huge expansion in its very early life, a period during which much of the matter of the universe and the energy for the explosion in the classical Big Bang was produced.

interference A characteristic property of waves whereby a new wave is formed when two waves overlap. When the two interfering waves are in step, an amplified wave motion is produced (constructive interference); otherwise, an attenuated wave motion is obtained (destructive interference).

interstellar medium The gaseous and dusty matter present in regions between a galaxy’s stars.

laser Acronym for Light Amplification by Stimulated Emission of Radiation. It is a device composed of an optically active medium, a pumping mechanism to inject energy into the medium and a resonant cavity to amplify the radiation. It can generate an intense directional beam of coherent light.

lepton Generic name for a class of fermions (electron, muon, tauon and neutrinos) that respond to weak and electromagnetic forces, but not to the strong force.

light shift (also called lamp shift) The shift of energy of an atomic level due to its local interaction with light. The shift depends on the intensity and the polarization of the light and can, therefore, be made to vary in space, e.g., periodically for a standing-wave of light. This is made use of in Sisyphus cooling.

lithography Production process in micro-electronics in which a pre-designed pattern is transferred from a template to a target material by exposing a selectively resistive overlay to energetic photons, electrons, ions or atoms. The patterned structure is then coated with a metal, infused with a dopant or subjected to further etching in order to fashion useful devices.

magnetic monopole A particle with a non-zero magnetic charge. All the known macroscopic and microscopic magnetic objects are magnetic dipoles, with equal and opposite magnetic charges which total to zero. A magnetic

monopole has never been found, but is postulated to exist in certain theories.

magneto-optical trap (MOT) A combination of laser Doppler cooling and electromagnetic trapping for neutral atoms consisting of three pairs of mutually perpendicular, appropriately polarized counterpropagating laser beams and a static magnetic-field configuration that provides a scattering force which depends on the position (for trapping) and velocity (for cooling) of the atoms. MOT is the workhorse for most BEC research work.

maser Acronym for Microwave Amplification by Stimulated Emission of Radiation, the immediate predecessor of the laser and operating along the same principles in the microwave band.

Meissner e ect Exclusion of magnetic flux from the bulk of a superconductor on cooling through the transition temperature. Hence, a superconductor is a perfect diamagnet.

meson Bosonic particle composed of a quark and an antiquark. Examples are the π-meson and the K-meson.

microwave background radiation Thermal radiation with a temperature of about 3 K uniformly distributed in the universe. The radiation is believed to be the cooled remnant of the hot radiation that was in thermal equilibrium with matter during the very early phases of the existence of the universe.

muon One of the three known kinds of charged leptons, a heavier analogue of the electron.

nanoscience Science dealing with extremely small objects (atomic-sized structures and devices, finely-structured materials, etc.).

nanotechnology Creation of functional materials, devices, and systems through control of matter at the scale of 1–100 nanometers, and the exploitation of novel properties and phenomena at the same scale.

neutrino Electrically neutral lepton. Its mass is either small or vanishing. There are three known varieties, one associated with each of the charged leptons (electron, muon and tau); their presence is always indicative of the action of the weak force.

neutron star A dense compact star consisting predominantly of neutrons and supported against gravity by degenerate-neutron pressure.

nonlinear optics The study of optical properties of matter under intense radiation fields. It is found that in such circumstances, the optical response of matter is nonlinear, i.e., strongly sensitive to the applied field, and manifests itself in some unusual forms, such as a nonlinear induced polarization, frequency mixing, harmonics generation and so on.

nuclear fission Division of a heavy-mass atomic nucleus into two nuclear fragments of nearly equal masses and possibly lighter particles, accompanied by a release of energy. This process is the operational basis of nuclear fission reactors and atomic bombs.

nuclear fusion Combination of two very light nuclei into a nucleus lighter than iron. This process is the main source of energy of stars, and also forms the operational basis of controlled fusion reactors and thermonuclear weapons. Also called nuclear burning.

nucleosynthesis Synthesis of chemical elements in the universe. Very light elements are synthesized during the hot early phases after the Big Bang. Other elements, not heavier than iron, are produced by nuclear fusion in the central regions of stars. Heavier elements are produced mainly during supernova explosions.

optical chaos An uncontrollable and unpredictable response exhibited by many nonlinear optical media under an applied intense field.

optical molasses A set of three pairs of counter-propagating laser beams along three mutually perpendicular directions, red-detuned so as to slow down, through the Doppler cooling e ect, an atom moving in an arbitrary direction. The retarding force is proportional and directed opposite to the atom’s velocity, making the system act as a viscous fluid — hence, the optical molasses.

order parameter A physical quantity that is a measure of the order of a thermodynamic state (or phase). It can be scalar, vector, or tensor. Thus, magnetization is the order parameter for a ferromagnet.

parity Left–right symmetry with respect to mirror reflection.

Pauli’s exclusion principle A general rule of quantum theory (found by Wolfgang Pauli) that forbids two fermions of the same kind to occupy the same quantum one-particle state in a given system. At any space-time point, at most one such particle can carry a given complete set of quantum numbers.

phase The phase of a wave is a measure of the state of the wave’s motion. When a wavelength of a wave has passed at a given point in space, we may say that the wave motion has completed a cycle, or that it has changed its phase by 360◦. Two waves are said to be in phase if their peaks coincide; otherwise, their phase di erence indicates how far apart their peaks are. In a completely di erent usage, the thermodynamic state of a macroscopic system is also called a phase. Thus, ice, water and steam represent di erent phases of H2O.

phase transition Change of thermodynamic state as some parameter, e.g., temperature is varied. First-order transitions, like freezing/melting, are discontinuous and involve latent heat; second-order transitions, like magnetic transitions, are continuous and have no latent heat.

photon Massless spin-one boson and quantum of the electromagnetic force. Planck constant The quantum of action, a fundamental physical constant

symbolized by h or = h/2π and named after Max Planck. It corresponds to the scale at which quantum e ects become important.

planetary nebula An expanding cloud of gas and matter around a hot star at a late evolutionary stage.

material; it is also a measure of this e ect, i.e., the electric dipole moment per unit volume. Dielectric polarization may arise from the distortion of the electron distribution about the nuclei in an electric field, or from a change in dipole moment resulting from the stretching of electrical bonds between unlike atoms in molecules.

polarization (of waves) Phenomenon observed when a transverse wave is polarized, that is, when the displacement of vibrations is completely predictable. A transverse wave is said to be unpolarized when the vibrations in the plane perpendicular to the direction of propagation appear to be oriented in all directions with equal probability; no preferred pattern of orientation can be observed over a long time period.

proton proton chain The sequence of nuclear fusion reactions that transform four protons into an α-particle in the core of ordinary stars.

pulsar A rotating neutron star emitting electromagnetic radiation, which astronomers detect in regular pulses.

quantum chromodynamics (QCD) The theory of strong interactions between quarks, in which the forces are mediated by gluons and the states characterized by color charges.

quantum electrodynamics (QED) The quantum theory of electromagnetic interactions between charged particles.

quark Fundamental particle of all hadrons. There are six known flavors of quarks, which combine in twos (a quark and an antiquark) to form mesons, and in threes to form baryons.

quasar Extremely bright source of radiation, thought to be a spinning black hole with an accretion disk from which radiation is emitted.

radiowaves Electromagnetic radiation of wavelengths 10−3–104 m. Radio astronomy is the branch of astronomy devoted to the radio observations of celestial objects.

renormalization A procedure to rewrite a fundamental physical law in an e ective form appropriate to a certain energy scale µ in terms of a number of measurable parameters.

renormalization group A mathematical relationship describing the fact that the arbitrary choice of µ should not a ect the physical outcome of a measurable quantity.

RR Lyrae stars A class of variable stars that can be used to determine astronomical distances.

scale invariance Symmetry demanding invariance with respect to a change of scale (of length, energy, etc.). Thus, a system at its critical point is scale invariant, whence all microscopic length scales, such as lattice spacing, become irrelevant.

self-assembly A process in which atoms, molecules or components arrange themselves into functioning entities without human intervention. Once launched, it is driven either by the laws of thermodynamics toward an energetically stable form or by built-in sets of instructions toward a pre-designed form.

Sisyphus cooling An ingenious mechanism devised for laser cooling based on repeated cycles of absorption and re-emission so arranged as to make the atom always move up-the-potential-hill, and hence continuously slow down. Here in each cycle of absorption and re-emission the atom switches between two ground-state sub-levels which are light-shifted periodically in space in antiphase, such that the crest of one coincides with the trough of the other.

spacetime The four-dimensional space in which the three spatial coordinates and the time coordinate are treated on an equal footing, as required by the special and general theories of relativity.

special relativity The currently accepted description of space, time and motion formulated by Einstein in 1905. A key result of the theory is no material body or physical signal can exceed the speed of light.

spin Intrinsic angular momentum of a particle or system of particles. It is characteristic of the particle and independent of its motion. As a quantum entity, it can take on integral or half-integral values in units of . According to the Theorem of Spin and Statistics, particles with integral spins are bosons, and particles with half-integral spins are fermions.

spontaneous emission Emission of a photon by an excited molecule or atom that is independent of external radiation.

spontaneous symmetry breaking The symmetry of the state becoming lower than that of the governing law. This takes place at a phase transition.

state function A mathematical object that contains the most complete physical information about the physical state of a quantum system.

stimulated emission Emission of a photon by an excited atom or molecule under the influence of an external field.

strange attractor A region of phase space (attractor) to which the phase trajectories are attracted but on which they wander aperiodically. It has fractal geometry and describes deterministic chaos.

strangeness A quantum number measuring the number of strange quarks inside a hadron.

string theory A theory which holds that the elementary particles are di erent manifestations of the excited modes of a tiny string, of the order of 10−35 m in size, and that all the four fundamental forces are produced by the joining and breaking up of the strings.

superconductivity A low temperature phenomenon of zero electrical resistivity and perfect diamagnetism shown by many materials below their characteristic critical temperatures.

superfluidity Total loss of viscosity exhibited by helium close to the absolute zero of temperature, which enables it to flow through the finest capillaries.

symmetry Invariance of an object with respect to certain operations like mirror reflection. The object may be an equation expressing physical law.

supernova The explosion of a massive star toward the end of its evolution, which leaves behind a supernova remnant composed of a gaseous expanding nebula and, in most cases, a central collapsed object which evolves eventually into a compact object.

tau lepton Negatively charged lepton, similar to but heavier than both the electron and the muon.

thermal convection Transfer of heat by moving masses of matter in gases and liquids.

thermal decomposition Process in which a metal-bearing precursor, when submitted to a high temperature and refluxing in a high-boiling point solvent, decomposes and combines with other atoms in the solvent to form crystallites.

top-down nanotechnology Makes existing structures or devices smaller. See lithography, gas-phase condensation.

truth One of the quark flavors; same as top.

tunnel e ect A quantum property which allows particles to pass through regions which are energetically forbidden by classical physics.

turbulent flow Fluid flow in which the velocity at any point of the fluid changes constantly in magnitude and direction, to be contrasted with a laminar flow, which is characterized by a regular space and time dependence.

uncertainty relations See Heisenberg’s uncertainty principle.

universality The assertion that the critical behavior of a system (i.e., behavior close to second-order phase transition) is independent of the behavior of the microscopic details and depends only on symmetry and dimensionality.

valence band The highest completely filled band of energy levels in a solid. virtual particle A quantum particle that exists only for very short durations and,

because of Heisenberg’s uncertainty relations, need not satisfy the usual relations between mass, energy and momentum, in contrast to the more familiar, long lived, real particles. Virtual particles may appear singly when exchanged between other particles, or in particle–antiparticle pairs when spontaneously created in vacuum.

weak interaction One of the four fundamental interactions. The weak force is responsible for β-decay and any interaction involving neutrinos or antineutrinos.

white dwarf A dense compact star with mass less than 1.4 solar mass, supported against gravity by degenerate-electron pressure.

X-rays Electromagnetic radiation with wavelengths around 10−9 m, or energies around 1 keV. X-ray astronomy is the study of celestial X-ray emitters, which include stars, supernovae and active galaxies.

X-ray binary system Double-star system that emits X-rays. Such systems consist of a normal star and neutron star or black hole that accretes matter from its companion star.

zero-point motion Quantum vibrational motion that is always present in a system, even at absolute zero temperature. The energy due to these quantum fluctuations is a minimum, irreducible amount of energy called the zero-point energy. Its existence may be regarded as a consequence of Heisenberg’s uncertainty principle.